I decided to discuss and rate the new cards from an Arena perspective, sorting them into five categories: Godly, Strong, Average, Bad, Terrible. I would consider a generic no-text 2-drop an average card.
Druid:
Enchanted Raven (Common): 1 mana, 2/2, Beast.
It's strong. Druid can contest the early game significantly better with a common class card like this. It trades with 3/2s and your hero power protects it from 2/1s. Beast synergy gives it late game relevance which 1-mana cards usually lack - consider potential turn 6 Enchanted Raven -> Druid of the Fang plays. I don't think it'll be super insane and turn Druid into an elite class though.
Menagerie Warden (Common): 6-mana, 5/5, Battlecry: Choose a friendly Beast, summon a copy.
Going with strong. You only pay 1-mana for the battlecry (a vanilla 5/5 for 5 is fine), and it's potentially game-winning. You'd definitely rather draft it in decks where you already have Beasts and have a fairly low curve, but Druid already has a ton of Beasts to pick from and it's curve will get lower in this adventure. I particularly like the thought of T5 Stranglethorn Tiger -> T6 Menagerie Warden, meet my three 5/5s. It's also the kind of card that will wreck games in the first week until people start playing around it, like Murloc Knight once did.
Moonlands Portal (Rare): 6-mana spell, Restore 6 health, summon a random 6-cost minion.
Strange card, but I'm going with strong. A random 6-cost minion is likely to be pretty good according to the spreadsheets - apart from Corrupted Seer there are very few truly awful outcomes, so you're likely giving up 1-mana to heal for 6. You could use that on your face against an aggro deck, but more likely you heal your Druid of the Claw or Druid of the Fang and start to get way ahead on the board. Flexibility is a big deal in Arena, and that's what carries this card into the strong tier I think.
Overall: Druid's moving up in the world. It's going to be more likely to take the board in the early game and if it gets to turn 6 with a board, expect it to race ahead. On the other hand, it still lacks the endless removal of Mage and the crazy tempo plays Rogue can pull off, so I see it placing 3rd at best when everything shakes out.
Hunter:
Kindly Grandmother (Common): 2 mana, 1/1 Beast, Deathrattle: summon a 3/2 Beast.
Think a lot of people are overshooting this a tad, but still strong. It gives Hunter more Beasts on the board, which is crucial, but it is a bit on the slow side. The opponent can choose to ignore it if they can't deal with the 3/2 and risk the potential Houndmaster. It also matters that unlike Haunted Creeper the first form dies to a ping, which gives Rogue and Mage more potential ways to clear it. Still, board presence is crucial for Hunter, and it will rightly be drafted often.
Cloaked Huntress (Common): 3 mana, 3/4. Your secrets cost 0.
It's strong with no text. You don't draft too many secrets in Arena, but maybe with this as a Hunter common you start to prioritise them. I'd consider it better than Kirin Tor as you'd normally rather have a 3/4 than a 4/3 on turn 3, and as a common your odds of drafting it to follow up an early Freezing Trap, say, are quite high.
Cat Trick (Rare): 2 mana, secret. When your opponent casts a spell, summon a 4/2 Panther with stealth.
Kind of a new mechanic, but I think it's bad in Arena. You get a 1-mana discount on a normal Jungle Panther, but you don't control when, if ever, you'll get it out.
Overall: Good boost to Hunter's early game, with a sticky 2-drop and a strong 3-drop, but not enough to move them up much.
Mage:
Firelands Portal (Common): 7 mana, spell. Deal 5 damage, summon a random 5-cost minion.
Going with a Godly here. 7-mana's a fair commitment, but for your money you get a strong removal that also summons a likely decent minion. It's essentially a built in 2 for 1 that doesn't cost you much, if any, tempo. Can also be used for extra reach, which Mage already has a ton of. The subtle effect of this card is to put even more pressure on opponents going into T7 - they have to decide which, both, or neither of Firelands Portal and Flamestrike to play around. The big thing is that if they play into a Firelands, they completely lose tempo - it kills their minion and establishes a board. Get ready to see this all the time.
Medivh's Valet (Common): 2 mana, 3/2. Battlecry: If you control a secret, deal 3 damage.
Higher end of average. You'll draft it, but you'll never be excited about it. Well, unless you live the dream, which might not be as rarely as you'd think. Hmm.
Babbling Book (Rare): 1 mana, 1/1, Battlecry: Add a random Mage spell to your hand.
Going with average. 1-mana 1/1s aren't anything special, but backed up with the threat of a ping on turn 2 they become more effective, and there are a lot of strong Mage spells. When I first saw this card, I thought it was 'discover' a spell, but this is significantly worse since you can't pick and there are quite a few dud Mage spells. Can also use it to fish for answers later in the game, so it retains relevance unlike a lot of early junk.
Overall: No one's messing with Jaina in her playground.
Paladin:
Nightbane Templar (Common): 3 mana, 2/1. Battlecry: If you're holding a dragon, summon two 1/1s.
It's bad. You don't hold dragons often enough to get the battlecry. Even with it it's not insane.
Silvermoon Portal (Common): 4 mana, spell. Give a minion +2/+2 and summon a random 2-cost minion.
Going with average. 4-mana is a crowded spot in Paladin where you want to playing Truesilver or Blessing of Kings, or elite 4-drops. This just isn't as good as BoK, nor is it even that close - you give up half the effect for a random 2-cost minion, many of which are straight up worse than an average 2-drop. And half the effect is a big deal - even if your minion can trade up with a 2/2 boost, it's less likely to hang around as a serious threat that doesn't die to a ping. From a more meta perspective, it lets Paladin do stuff it already does - buff minions - so it's even more incentive for opponents to play correctly and keep the board clean.
Ivory Knight (Rare): 6-mana, 4/4. Battlecry: Discover a spell. Restore HP to your hero equal to its cost.
Average. Gets you card advantage, but it's slow, and a 4/4 on 6 just isn't good enough to properly contest the board. Many Paladin spells are just not too good either - there's the secrets, then fluff like Anyfin Can Happen, before you get to your Consecrations. And a lot of the good spells buff your board, which you might not have after such a slow play.
Overall: Paladin got the short end of the stick this expansion. One common is bad, and the other does nothing new. Down we go.
Priest:
Purify (Common): 2-mana, spell. Silence a friendly minion. Draw a card.
Oh, Priest. Terrible.
Priest of the Feast (Common): 4 mana, 3/6. Whenever you cast a spell, restore 3HP to your hero.
Average. It's a fine vanilla minion, but 3 attack is a bit low to really contest bigger threats. If you have the board going into T4, it could be the card that leads to snowballs, but you're Priest, so yeah.
Onyx Bishop (Rare): 5 mana, 3/4. Battlecry: Summon a friendly minion that died this game.
Probably lower end of strong. The effect is good, but it's slow and value oriented. Priest doesn't have problems winning the long war as it is. But it will roll huge and win some games, and even when it misses it isn't going to be much worse than a Silver Hand Knight.
Overall: Time to start up #ArenaPriestsMatter.
Rogue:
Swashbuckler (Common): 1 mana, 1/1. Battlecry: Add a random class card (opponent's class) to your hand.
Strong card here. 1-mana cards are always artificially boosted in Rogue due to Combo, and getting a draw is good. Nothing crazy though - 1/1 for 1 is nothing to get excited about and the card's average value will only be above average at best, depending on the class. Expect to see it all the time.
Deadly Fork (Common): 3 mana, 3/2. Deathrattle: Add a 3/2 weapon to your hand.
I wrote out a big paragraph explaining why this card was insane, only to realise you need to pay 3 further mana to play the weapon. That puts it squarely in average territory. It's slow, going on slower, but it will eventually give some card advantage back.
Ethereal Peddler (Rare): 5 mana, 5/6. Your cards from other classes cost 2 less.
Godly. It's a Pit Fighter with massive synergy with Burgle, Undercity Huckster and Swashbuckler. Rogue already has no problems getting ahead in tempo most of the time; when this combines with strong pulls from the above cards things are going to get crazy real fast. It also has a massive bullseye on its back for that reason, but that can be used to a smart player's advantage.
Overall: Rogue gets one strong common and one potentially crazy rare that acts as another big dude for a class that traditionally lacked big dudes. Also gets a bit of a dud, so likely to stay where it is.
Shaman:
Wicked Witchdoctor (Common): 4 mana, 3/4. Whenever you cast a spell, summon a random basic totem.
Bad. Understatted and you need to follow up with spells to even try to catch up.
Spirit Claws (Common): 1-mana, 1/3 weapon. Has +2 attack while you have spell damage.
Forget the spell damage and this is a bone average card. Likely get value thanks to the prevalence of 1HP minions, but you also make playing other weapons awkward. Rare decks with good spell damage minions will see this get crazy value.
Maelstrom Portal (Rare): 2-mana, spell. Deal 1 damage to all enemy minions, summon a random 1-cost minion.
Strong. It's Arcane Explosion's big brother, and that's a decent card with so many tokens around as is. Very nice to have a Shaman AoE that doesn't overload so you can actually build a proper tempo edge. (With spell damage this is a 2-mana Consecration with upside, and will win games.)
Overall: I guess they want Shamans to draft more towards spell damage? At least there's no overload to deal with, but the two commons aren't exactly game changing.
Warlock:
Malchezaar's Imp (Common): 1-mana, 1/3. Whenever you discard a card, draw a card.
It's average, but it fits Warlock's Zoolock-esque game plan - get on the board and never lose it.
Kara Kazham! (Common): 5-mana, spell. Summon a 1/1, 2/2 and 3/3.
Would lean average here. It's a Silver Hand Knight with 3 bodies and the stats redistributed to compensate, but it's the 4/4 that does most of the work for SHK, and it's easy to see at least one or two of these dudes being brushed aside. Really pushes a Zoolock style approach, with cards like Sea Giant, Frostwolf Warrior et. al. that benefit from flooding.
Silverware Golem (Rare): 3-mana, 3/3. If you discard this minion, summon it.
Text is irrelevant almost always, so this is bone average.
Overall: Nothing to get excited or angry about. Both commons push Warlock to go more for taking the board, spreading wide and going for it, so meta changes will be important for the class.
Warrior:
Fool's Bane (Common): 5-mana, 3/4 weapon. Unlimited attacks each turn, but can't go face.
Clearly strong. You play it and hopefully clear the board in exchange for a chunk of HP/Armor. If Assassin's Blade's main drawback is that it's too slow, this card's only drawback is the face risk. You quickly gain card advantage and tempo, and in many matchups that will be enough to win the game. Definitely seems like a card that will have a big difference in performance between the best players and the average.
Ironforge Portal (Common): 5-mana, spell. Gain 4 Armor, summon a random 4-cost minion.
Bad. You pay 5 mana to get a below average 4 drop and the Armor isn't going to make up the value lost.
Protect the King! (Rare): 3-mana, spell. For each enemy minion, summon a 1/1 with Taunt.
Strange card. My immediate thought is bad. You usually won't see more than three minions on the board in Arena, and in those cases you're likely too far behind for this to matter. Too reactive.
Overall: Warrior gets one strong common and one poor one. There's enough here for the warrior specialists to work with, but nowhere close to enough for anyone else to jump in enthusiastically.
Neutrals:
Pompous Thespian (Common): 2-mana, 3/2, Taunt.
It's average. Occasionally the Taunt will come in handy, but mostly it just makes it more likely everyone curves out.
Zoobot (Common): 3-mana, 3/3. Battlecry: Give a random friendly Beast, Dragon and Murloc +1/+1.
Leaning strong. Beasts in particular are common, and Dragons and Murlocs aren't so rare you won't have 1 or 2 in your deck. But you need to have the board to get extra value, or play it after a small tribe card. Obviously better in Druid and Hunter, where it's probably a priority pick.
Netherspite Historian (Common): 2-mana, 1/3. Battlecry: If you're holding a dragon, discover a dragon.
It's bad. You don't hold dragons often enough to get the discover with any regularity, and if you don't hit it, it's a terrible card.
Runic Egg (Common): 1-mana, 0/2. Deathrattle: Draw a card.
Terrible. It's for constructed. You're paying a mana and a card to drop a useless body on the board that will die hopelessly and set you behind. Might be better than absolute trash in Rogue, but otherwise won't be played.
Pantry Spider (Common): 3-mana, 1/3, Beast. Battlecry: summon a 1/3 Spider.
Bad. It's technically 2/6 on turn three, but one half of it dies hopelessly against anything you're likely to see on turn 3, while the other half limps on. Good to play on T3 ahead of Houndmaster in Hunter though.
Arcane Anomaly (Common): 1-mana, 2/1. Whenever you cast a spell, it gains 1HP.
First thought was bad, but on reflection going average. There's enough low cost spells in the game that I can see it squeaking out some value with Coin -> Journey Below or Coin -> Innervate type plays. Generally though, vanilla 2/1 is just not good enough (see Murloc Raider).
Menagerie Magician (Common): 5-mana, 4/4, give a random friendly Beast, Dragon and Murloc +2/+2.
Difficult card to rank, because it should really only be drafted when you have the synergy set up - without the effect it is plain bad. But with even one activation, it's strong. At 5-mana though, it's not going to be easy to play it with another minion, so you'll need to have the board for it to be useful, and people should know to keep tribes off the board in the new expansion thanks to this kind of card. Going strong, especially in Druid and Hunter.
Arcanosmith (Common): 4-mana, 3/2. Battlecry: Summon a 0/5 Taunt.
Bad. Shieldbearer isn't worth it at 1 mana, so paying 2 mana for an extra HP doesn't strike me as especially appealing. I'm sure people will persevere though and try to get Blessing of Kings (or equivalent) off on the 0/5 Taunt, and it'll come off occasionally.
Violet Illusionist (Common): 3-mana, 4/3. On your turn, your hero is immune.
In non-weapon/Warlock classes, it's a perfectly above average 3-drop. But where you can get value from the text it's going to be much stronger than people expect - consider hacking through a high attack minion with your face as Rogue or Warrior, or dropping an otherwise unplayable Flame Imp in Warlock. Strong in those classes.
Book Wyrm (Rare): 6-mana, 3/6. Battlecry: If you're holding a dragon, destroy an enemy minion with 3 or less attack.
Reminds me of Blackwing Corruptor, but with less useful stats. With dragons being as rare as they are, I'm edging bad - the stats aren't good for a 5-drop, never mind a 6, so you need the battlecry. Definitely a strong pick with enough dragons though.
Avian Watcher (Rare): 5-mana, 3/6. Battlecry: if you control a secret, gain +1/+1 and Taunt.
Average. You don't control secrets near often enough to make the battlecry likely, and without it it's Fen Creeper with no Taunt, which isn't good. Even with it, it's not a huge deal.
Moat Lurker (Rare): 6-mana, 3/3. Battlecry: Destroy a minion. Deathrattle: Resummon it.
Going with bad. You pay a massive price for the effect, which is rarely going to be worth it. If you're a super-aggro deck maybe you can use it to end the game, but otherwise they just kill your 3/3 for 6 and get their minion back. You can be cute and trigger your own deathrattle with it, which might get value occasionally, but isn't a reliable play.
Arcane Giant (Epic): 12-mana, 8/8. Costs 1 less for each spell you've cast this game.
Bad. You need to cast 4+ spells to make this remotely worth it. You'll know if you have the rare deck where this might be worth it, but they will be very rare.
Barnes (Legendary): 4-mana, 3/4. Battlecry: Summon a 1/1 copy of a random minion in your deck.
Chances are your random minion doesn't have exciting text in Arena. So I'd go with bad.
The Curator (Legendary): 7-mana. 4/6, Mech. Taunt. Battlecry: Draw a Beast, Murloc and Dragon from your deck.
That's some serious card advantage if you have good cards from all three tribes. Even just two and it's likely better than old Ancient of Lore. Going with strong, but with the proviso that you need to actually have some good cards from those tribes in your deck.
Moroes (Legendary): 3-mana, 1/1. At the end of your turn, summon a 1/1.
Gut reaction is bad. 3-mana for two 1/1s is awful. Even if it lives two turns you'd struggle to make up the tempo loss. But in decks that can do wonders with tokens it might be worth it. Usually bad.
Prince Malchezaar (Legendary): 5-mana, 5/6, Demon. At the start of the game, add 5 Legendary minions to your deck.
Strong. It's a Pit Fighter with upside, and a tribe that makes it even better in Warlocks. 5 Legendaries might contain some rubbish, but the average Legendary is a perfectly playable card and several of them are crazy, and having 35 cards can matter, especially in Warlock.
Medivh, the Guardian (Legendary): 8-mana, 7/7. Battlecry: Equip Atiesh (1/3, when you cast a spell, summon a random minion of that cost. Lose one durability).
Going with strong. If you don't have a spell it's an overcosted War Golem, which isn't good, but is still a card, and the weapon might help out a little. But where you can follow up with big spells this card is going to dominate the late game. Somewhere a Priest is going to desperately salvage a game with Medivh -> Mind Control.
Overall ratings (excludes the Epic and the 5 Legendaries):
Godly: 2/39 (Firelands Portal, Ethereal Peddler)
Strong: 12/39 (Enchanted Raven, Menagerie Warden, Moonlands Portal, Kindly Grandmother, Cloaked Huntress, Onyx Bishop, Swashbuckler, Maelstrom Portal, Fool's Bane, Menagerie Magician, Zoobot, Violet Illusionist)
Average: 13/39 (Medivh's Valet, Babbling Book, Silvermoon Portal, Ivory Knight, Priest of the Feast, Deadly Fork, Spirit Claws, Malchazaar's Imp, Silverware Golem, Kara Kazham!, Pompous Thespian, Arcane Anomaly, Avian Watcher)
Bad: 10/39 (Cat Trick, Nightbane Templar, Wicked Witchdoctor, Ironforge Portal, Protect the King!, Netherspite Historian, Pantry Spider, Arcanosmith, Book Wyrm, Moat Lurker)
Terrible: 2/39 (Purify, Runic Egg)